1. Field of the Invention
The present invention relates to hydraulic systems for operating machinery, and in particular to control algorithms for electrically operating valves in such systems.
2. Description of the Related Art
A wide variety of machines have moveable members which are operated by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. Traditionally the hydraulic valve was manually operated by the machine operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid operated valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near an operator station, but can be located adjacent the actuator being controlled. This change in technology also facilitates sophisticated computerized control of the machine functions.
Application of pressurized hydraulic fluid from a pump to the actuator can be controlled by a proportional solenoid-operated valve. This type of valve employs an electromagnetic coil which moves an armature connected to a valve element, such as a spool or poppet for example, that controls the flow of fluid through the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the fluid flow. Either the armature or the valve element is spring loaded to close the valve when electric current is removed from the solenoid coil. Alternatively, another electromagnetic coil and armature is provided to move the valve element in the opposite direction.
When an operator desires to move the member on the machine, a joystick is manipulated to produce an electrical signal indicative of the direction and desired rate at which the corresponding hydraulic actuator is to move. The faster the actuator is desired to move, the farther the joystick is moved from its neutral position. A control circuit receives a joystick signal and responds by applying an electric current to the electromagnetic coil which opens the valve by an amount that results in a rate of fluid flow which produces the desired motion of the hydraulic actuator.
Key to the operation of the solenoid-operated valve is the ability of the control circuit to produce the correct magnitude of electric current to open the valve to the proper degree.